(1900) The sphinxèd riddle of the Universe / Nature's unsolved enigma, who may prove?

(1933) Janet Gaynor, so they tell, is sphinxing it and has gone into a Retirement, with "Nothing to Say — Please Go Away" written on the doormat.

(1934) The men of science will climb grassy hillsides of [Easter] island to peer at hundreds of great stone faces that have so far out-sphinxed the sphinx in determined silence about the past.

(1954) "What are you two sphinxing about?" said Jessica, but her husband said Never mind

(1964) What with Fisher whole-hogging on one side, and K. of K. sphinxing on the other, Churchill had his work cut out to get any sort of agreement at all.

To make one guess at the unknowable

(1933) Perhaps Nature is sphinxing us on purpose. Whatever her objects may be, perhaps she gets her work done better when she appeals to our gambling instincts. If you knew for certain exactly how your marriage was going to turn out ...